1. Field of the Invention
The invention concerns a device for adjusting the clearance between the moving blades and the ring of a turbine and is designed to maintain a reduced and essentially constant clearance during changes in turbine speed, with such device including an inner sleeve having a seal, a perforated, cylindrical partition encompassing the sleeve, and an outer sleeve defining an enclosure which receives the air intended to heat or cool the ring after passing through the perforated partition.
2. Description of the Prior Art
The efficiency of a turbine is a function of a number of parameters, particularly the clearance existing between the tips of the blades and the stator. This clearance is set during construction at a given low value, and in order to avoid accidental rubbing during rotation, the turbine ring is generally provided with a seal made of abradable material which allows for non-destructive contact with the blades. Such rubbing results from differences in thermic expansion between the turbine disks and blades on the one hand and the housing which supports the ring on the other. The clearance provided in construction thus varies with the rapid changes in speed and temperature of the turbine.
In the starting and acceleration phases, the turbine blades and ring heat up more quickly than the disk, which produces an expansion of the ring and an increase in the clearance between the blades and the ring. In the deceleration phase, the blades and ring cool more quickly than the disk and clearance is minimal, with the risk of interference between blades and ring. In order to minimize, if not eliminate, variations in clearance, manufacturers have sought to made dimensional variations of the rotor and stator simultaneous through the selection of material expansion coefficients and through control of the temperature of the ring or of the structure which supports it.
To this end, French Pat. No. 2,064,889 describes a seal ring held in place by an annular support. This support communicates with pressurized air from the compressor and includes a flange having a large thermic mass. Passages provided in the wall of the support direct air toward the flange into a chamber which is also closed by a perforated wall. This perforated wall forms a second chamber in conjunction with the wall of the ring. The pressurized air serves to heat or cool the ring support. Then the same air is used to heat or cool the ring itself through the formation of jets across the perforated wall of the second chamber, ensuring a high speed of heat transfer between the air and the ring. Because the ring is segmented and held in place by flanges disposed at its two ends, the risk of non-simultaneous expansion of the extreme parts is not excluded. The connections between the segments and supports do not provide a suitable seal and the escape of gas makes temperature control difficult. Furthermore, mechanical assembly is complicated, which has the consequence of causing relatively long down-times during repair of the ring.
French Pat. No. 2,293,594 describes a device in which the seal ring, consisting of segments comprising protrusions and flanges, is held by an annular element supported by studs fastened at their outer end in holes provided in the envelope. This annular element includes holes enabling the passage of high pressure air from the compressor. A second annular element, having a greater mass than the first, is insulated from the high-pressure air by a screen. During speed variations, the second solid annular element, protected by the screen, expands or contracts less quickly than the first, thus enabling control of the expansion of the ring support and consequently the maintenance of clearance. The drawbacks of this construction are essentially the same as those pointed out for the first patent cited.